The desirability and effectiveness of well logging systems where information is sensed in the well hole and transmitted to the surface through mud pulse telemetry has long been recognized. Mud pulse telemetry systems provide the driller at the surface with means for quickly determining various kinds of downhole information, most particularly information about the location, orientation and direction of the drill string at the bottom of the well in a directional drilling operation. During normal drilling operations, a continuous column of mud is circulating within the drill string from the surface of the well to the drilling bit at the bottom of the well and then back to the surface. Mud pulse telemetry repeatedly restricts the flow of mud to propagate signals through the mud upward to the surface, thereby providing a very fast communication link between the drill bit and the surface. Depending on the type of drilling fluid used, the velocity may vary between approximately 3000 and 5000 feet per second.
A telemetry system may be lowered on a wireline located within the drill string, but is usually formed as an integral part of a special drill collar inserted into the drill string near the drilling bit. The basic operational concept of mud pulse telemetry is to intermittently restrict the flow of mud as it passes through a downhole telemetry valve, thereby creating a pressure pulse in the mud stream that travels to the surface of the well. The information sensed by instrumentation in the vicinity of the drilling bit is encoded into a digital formatted signal and is transmitted by instructions to pulse the mud by intermittently actuating the telemetry valve, which restricts the mud flow in the drill string, thereby transmitting pulses to the well surface where the pulses are detected and transformed into electrical signals which can be decoded and processed to reveal transmitted information.
Representative examples of previous mud pulse telemetry systems may be found in U.S. Pat. Nos. 3,949,354; 3,958,217; 4,216,536; 4,401,134; and 4,515,225.
Representative samples of mud pulse generators may be found in U.S. Pat. Nos. 4,386,422; 4,699,352; 5,103,420; and 5,787,052.
A telemetry system capable of performing the desired function with minimal control energy is desirable, since the systems are typically powered by finite-storage batteries. One such example is found in U.S. Pat. No. 5,333,686, which describes a mud pulser having a main valve biased against a narrowed portion of the mud flowpath to restrict the flow of mud, with periodic actuation of the main valve to allow mud to temporarily flow freely within the flowpath. The main valve is actuated by a pilot valve that can be moved with minimal force. The pilot valve additionally provides for pressure equalization, thereby increasing the life of downhole batteries.
Another example of an energy efficient mud pulser is described in U.S. Pat. No. 6,016,288, the mud pulser having a DC motor electrically powered to drive a planetary gear which in turn powers a threaded drive shaft, mounted in a bearing assembly to rotate a ball nut lead screw. The rotating threaded shaft lifts the lead screw, which is attached to the pilot valve.
Solenoid-type pulser actuators have also been used to actuate the main pulser valve, however, there are many problems with such a system. The use of a spring to bias the solenoid requires the actuator (servo) valve to overcome the force of the spring (about 6 pounds) and of the mud prior to actuating the main valve. A typical solenoid driven actuator valve is capable of exerting only 11 pounds of pressure, leaving only 5 pounds of pressure to actuate the pulser assembly. Under drilling conditions requiring higher than normal mud flow, the limited pressures exerted by the solenoid may be unable to overcome both the pressure of the return spring and the increased pressure of the flowing mud, resulting in a failure to open the servo-valve, resulting in the main valve remaining in a position in which mud flow is not restricted, and therefore failing to communicate useful information to the surface.
A further problem with the use of a solenoid to actuate the pulser assembly is the limited speed of response and recovery that is typical of solenoid systems. Following application of a current to a solenoid, there is a recovery period during which the magnetic field decays to a point at which it can be overcome by the force of the solenoid's own return spring to close the servo-valve. This delay results in a maximum data rate (pulse width) of approximately 0.8 seconds/pulse, limiting the application of the technology.
Moreover, the linear alignment of the solenoid must be exactly tuned (i.e. the magnetic shaft must be precisely positioned within the coil) in order to keep the actuator's power characteristics within a reliable operating range. Therefore, inclusion of a solenoid within the tool adds complexity to the process of assembling and repairing the pulser actuator, and impairs the overall operability and reliability of the system.
Existing tools are also prone to jamming due to accumulation of debris, reducing the range of motion of the pilot valve. Particularly when combined with conditions of high mud flow, the power of the solenoid is unable to clear the jam, and the tool is rendered non-functional. The tool must then be brought to the surface for service.
Stepper motors have been used in mud pulsing systems, specifically, in negative pulse systems (see for example U.S. Pat. No. 5,115,415). The use of a stepper motor to directly control the main pulse valve, however, requires a large amount of electrical power, possibly requiring a turbine generator to supply adequate power to operate the system for any length of time downhole.
Repair of previous pursers has been an as yet unresolved difficulty. Typically, the entire tool has been contained within one housing, making access and replacement of small parts difficult and time-consuming. Furthermore, a bellows seal within the servo-poppet has typically been the only barrier between the mud flowing past the pilot valve's poppet and the pressurized oil contained within the servo-valve actuating tool, which is required to equalize the hydrostatic pressure of the downhole mud with the tool's internal spaces. Therefore, in order to dissemble the tool for repair, the bellows seal had to be removed, causing the integrity of the pressurized oil chamber to be lost at each repair.
Furthermore, a key area of failure of MWD pulser drivers has been the failure of the bellows seal around the servo-valve activating shaft, which separates the drilling mud from the internal oil. In existing systems, the addition of a second seal is not feasible, particularly in servo-drivers in which the servo-valve is closed by a spring due to the limited force which may be exerted by the spring, which is in turn limited by the available force of the solenoid, and cannot overcome the friction or drag of an additional static/dynamic linear seal.
It remains desirable within the art to provide a pulse generator that has an energy efficiency sufficient to operate reliably and to adapt to a variety of hostile downhole conditions, has reduced susceptibility to jamming by debris, and is simpler to repair than previous systems.